Bismuth phosphate process for the separation of plutonium from aqueous solutions



Mrch 19, 1957 s. G. THOMPSON Erm. 2,785,951 BISMUTH PHOSPHATE PROCESSFOR THE SEPARATION OF PLUTONIUM FROM AQUEOUS SOLUTIONS Filed Jan. 26,1944 V f/IoJp/hzfe )Vrecg'azzazz I pfefzfafe jeff/m? i l rzzczzozz 4Prec-Dz'a/z/z j United States Patent() BlSMUTI-I PHOSPHATE PROCESS FOR'I HE SEP- ARATION F PLUTONIUM FROM AQUEOUS SOLUTIONS Stanley G.Thompson and Glenn T. Seaborg,y Chicago,

lll., assignors to the United States of America as represented `by theUnited States Atomic Energy Commission Application `lanuary 26, 1944,Serial No. 519,714

Claims. (Cl. 23-14.5)

The invention relates to the separation of element 94 from othersubstances and more particularly to the separation of element 94 fromsubstances of the kind present in neutron irradiated uranium, such asuranium, fission products, and the like, which are considered to beforeign porducts.

It is an object of the invention to obtain element 94 in a relativelypure state by a convenient process which is suitable for use on both acommercial and laboratory scale, and which isparticularly adapted forconcentrations of element 94 such as are obtained from neutronirradiated uranium Where fission products and other substances resultingfrom a neutron bombardment of uranium are present. Other objects andadvantages will be apparent from the following detailed description.

In the drawing, a diagrammatic representation of one embodiment of theinvention is given in the form of a flow sheet.

As described herein, the isotope of element 93 having a mass of 239 isreferred to las 93239 and the isotope of element 94 having a mass o'f239 is referred to as 94239. Element 94 may also be spoken of asplutonium, symbol Pu. Reference herein to any of the elements is to beunderstood as denoting the element generically, Whether in its freestate or in the form of a compound, unless indicated otherwise by thecontext.

Neutron irradiated uranium may be prepared by reacting uranium withneutrons from any suitable neutron source, but preferably the neutronsused are obtained Y from a chain reaction of neutrons with uranium.

Neutron irradiation of uranium produces 92U233 which has a half-life of23 minutes and by beta decay becomes 93233. This element has a half-lifeof 2.3 days and by beta decay becomes 94239. Neutron irradiated uraniumcontains 93239, 94239 and a large number of radioactive fission productsproduced by reaction of neutrons on fissionable atoms, such as U235which is present in uranium from natural sources. It also contains minoramounts of other products such as UX1 and UXz. The amount of 93239 and94239 combined is generally minute, such as, for example, approximately0.02% by weight. By storing the neutron irradiated uranium for fasuitable period of time, the 93233 is converted almost entirely to94329. The fission products are present in the neutron irradiateduranium generally to an extent of about 0.02% by weight. Because thefission products in general are highly radioactive, it is preferred thatthese materials be removed.

The ssion products consist of a large number of elements which may beclassified into two groups; a light group with atomic numbers from 35 to45; and a heavy group with atomic numbers from 51 to 60. The fissionproducts with which we are particularly concerned are those having ahalf-life of more than three days since they remain in the neutronirradiated reaction mass in substantial quantities at least one monthafter reaction. These products are chiefly Sr89, Y (57 day half-life),Zr, Cb, and Ru of the group of atomic numbers from 35 to 45; and Te12",Tem, 1131, Xe133, Cs (many years e ICC half-life), Ba (12 dayshalf-life), Lam, and Ce of 20 day and 200 day half-lives from the groupof 'atomic numbers from 51 to 60, inclusive.

With respect to the present invention, the foreign products, that is,the fission products, uranium, and other substances present in neutronirradiated uranium, may be classified into those foreign products whichare insoluble in the presence of phosphate ions and those which aresoluble in the presence of phosphate ions. While the concentrations ofcertain of the phosphate insoluble foreign products may be so low incertain instances to prevent them from being thrown out of solution asinsoluble phosphates, such foreign products may, nevertheless, becarried out of solution by other substances which are precipitated asphosphates.

In its broadest aspect, the invention lprovides a method for separatingplutonium from fission products which comprises separating such of thefission products as are in a phoshpate solubility state opposite to thatof plutonium and then converting plutonium to an opposite phosphatesolubility state and separating plutonium from those fission productsthat now have a phosphate solubility state opposite to that of theconverted plutonium.

The solubility of plutonium in the presence of phosphate ions is changedby changing the oxidation state of the plutonium, as it has beendiscovered that plutonium has more than one oxidation state; a loweroxidation state in which the element forms an insoluble phosphate in thepresence of phosphate ions; and also a higher oxidation state or statesin which the element does not form a phosphate insoluble in the presenceof phosphate ions. As the foreign products form substances which are inpart insoluble and in part soluble in the presence of phosphate ions,the process contemplates separating plutonium in its phosphate insoluble(lower oxidation) state from the phosphate soluble foreign products,then converting the plutonium contained in the phosphate insolubleportion to its higherV oxidation (phosphate soluble) state andseparating the latter from the remaining foreign products which are in aphosphate insoluble state. The soluble plutonium phosphate in whichplutonium is present in its higher oxidation state is referred to asplutonyl phosphate, (PuOz)3(PO4)2, in the claims appended to thisspecification. The above process, being based on oxidizing and reducingthe plutonium to change its solubility in the presence of phosphateions, mayV for convenience be referred to as an oxidation-reductionseparation. Such oxidation-reduction separation may be used as acomplete process for separating plutonium from fission products,uranium, or other foreign products. The steps of oxidation and reductionmay be employed in either sequence desired.

Where the plutonium is associated with substances of the kind present inneutron irradiated uranium, it is generally preferred to separate asmuch of the uranium as possible from the plutonium before treating theplutonium in accordance with the oxidation-reduction separation, as thepresence of uranium interferes with the reactions by which the plutoniumis separated, particularly if the uranium is present in concentrationswhich are relatively high as compared to that of plutonium. Variousmethods for partially separating the plutonium from uranium and fissionproducts may be used in conjunction with the oxidation-reductionseparation. A process which has been found to be particularly suitablefor use as a preliminary Astep in the oxidation-reduction separation ofplutonium is a phosphate precipitation step in which the plutonium isseparated as yan insoluble phosphate from substantially all, of thetitanium and major amounts of fission products. The partially purifiedplutonium is then further purified by being treated in accordance withthe oxidation-reduction separation which, when used with the phosphateprecipitation u step or other preliminary separation method, may beconsidered as an oxidation-reduction phase of the overall separatoryprocess.

lncarrying out the preliminary phosphate precipitation phase wheretheplutonium is present in concentrations above approximately 800 mg.per -liter of solution, the plutonium may be separated fromsubstantially all of the uranium and many of the fission productspresent in the solution by precipitating the plutonium and minor amountsof fission products as insoluble phosphates through the addition of anexcess of phosphate ions to the solution, which is adjusted toapproximately one normal acid content. If the solution contains anappreciable `amount of uranium, it may also be desirable to have presentions such as sulphate ions, phosphate ions at high'acid concentrations,or the like, which will form a complex coinpound of uranium and therebyprevent the uranium from precipitating as an insoluble phosphate. Inthis concentration, the plutonium is present above its limit ofsolubility and will be thrown down as an insoluble phosphate precipitatewhich is easily removed from the other'substanees in solution. Y

Such high plutonium concentrations may be present in neutron irradiateduranium, especially where the uranium is bombarded with neutronsobtained from a chain reaction for long periods of time. In extractingthe plutonium, the neutron irradiated uranium is dissolved in nitricacid and diluted with water to form a solution containing 20% uranylnitrate hexahydrate, UO2(NO3)2.6H2O, which Solution will also containthe plutonium, fission products, and other substances as nitrates. Thesolution is Vadjusted to an acid content of approximately one normal.Suiicient sulphate ions or other complexing ions are added to preventprecipitation of the uranium and other substances present, this beingdone by adding sulphuric acid, sodium sulphate, or other solublesulphate. The solution is preferably aciditied with H2804 which willalso introduce the necessary sulphate ions. An excess of phosphate ions,preferably in the form of phosphoric acid, is added to the solution toprecipitate the plutonium. The insoluble plutonium phosphate and minoramounts of fission products in the form of phosphates are thrown downand may be separated from the solution by any convenient means suchasriiltration, centrifugation, or the like.

Where the plutonium is present in relatively low concentrations as, forexample, mg. per liter of solution, as is usually the case in a solutionof neutron irradiated uranium, it is below its limit of solubility inthe solution so that the addition of phosphate ions will not cause theplutonium to be thrown down as a precipitate. To carry out thepreliminary phosphate precipitation phase with such low concentrationsof plutonium, it has been found desirable to use suitable substanceswhich will be converted into insoluble phosphate carriers by phosphateions and which will carry the plutonium phosphate out of solution. Suchcarriers must necessarily be phosphates which are relatively insolublein acid solutions in which uranium or uranyl phosphate is soluble, suchas, for example, bismuth phosphate and zirconium phosphate.

Bismuth phosphate, in particular, has been found to be very effective incarrying7 plutonium lout of solution as it has been discovered thatplutonium may be removed from the uranyl nitrate hexahydrate solution tothe extent of 95% or more where bismuth phosphate is present in bismuth:plutonium ratios varying from :1 to 200,000zl. The carrying elect thatbismuth phosphate exhibits on plutonium may be due in whole or part to asurface association with the plutonium such as adsorption, complexesformed Which are in the nature of compounds, or other types ofassociations. it is also possible that the reason for this behaviorresides in the fact that the bismuth phosphate precipitate isisomorphous with plutonium phosphate. Separation of the insolublephosphates from the solution by conventional means'such as filtration isfacilitated by the-crystalline structure of the precipitated bismuthphosphate. Thus, while many carriers may be suitable for carrying theplutonium phosphate from solution, preferably the carrier is one which,like bismuth phosphate, is isomorphic with plutonium phosphate. However,while We have reason to believe that the use o-f a carrier which isisomorphic with plutonium phosphate leads to particularly desirableresults, we do not wish to be bound by any theory as to the action ofthe carrier.

The following experiment and accompanying table is given to sho-w thatbismuth phosphate has a very high carrying power for element 94 inconcentrations ranging from slightly above tracer concentrations toconcentrations Where a carrier is not needed; that is, where theconcentration of plutonium exceeds the solubility of plutonium phosphatein the solution.

EXAMPLE I A uranyl nitrate solution containing 20% uranyl nitratehexahydrate, 25 mgm. Bit3 per l0 ml., and l N in HNOS was prepared. Tosmall volumes (100, l0 or l 10-3 ce.) of this was added appropriatequantities of Fut1 (as a solution of the nitrate) to obtain the desiredBizPu ratio. This was followed'by the addition of suiilcient 3.6 M HsPOeto makeV the solution 0.36 M H3PO4. The solutions were heated at C. for45 minutes and then centrifuged.V Aliquots of the supernatant liquidwere taken for alpha-counting. The precipitates were washed 3 times with.04 cc. of 3.6 M HaPOi, dissolved in concentrated HC1 and transferred toplatnium dishes for alpha-counting. The 94 in the aliquots of thesupernatant liquids was separated from uranium by LaFa precipitation;that is, by insuring the presence of the 94 in the reduced state, addinga solution containing lanthanum ion followed by the addition ofhydrofluoric acid solution and digesting for a relatively shortinterval, whereupon lanthanum fluoride precipitates and carries the 94out of solution. Results of the experiments are summarized in the tablebelow.

Table I Activity rc- Activity rccovered in the covered in the PercentRatio BizPu supernatant bismuth phos- 94 presolution 1 phateprecipicipitatcd (Counts per tate (Counts minute) per minute) 11, 92098. Q 92 6. 300 98. 5 92 13, 800 Q9. 4 184 12, 600 98. 5 l2 744 98. 4 6134 95. 5

1 These counts are corrected for a uranium blank by subtracting 18counts per minute obtained by precipitating 4 104 grams oi La as thcfluoride in the presence of uranyl nitrate containing no Pu.

2 The concentration of Pu in this experiment was such as to probablyexceed the solubility of plutonium phosphate in uranyl nitrate.

In accordance with one embodiment of the invention, neutron irradiateduranium containing element 94 inconcentrations above tracer amounts as,for example, concentrations of approximately V0.01 percent by weight ofelement 94 is dissolved in nitric acid `to form uranyl nitratehexahydrate, as Well as nitrates of plutonium, neptunium (element 93),and the various fission products. In any case, an excess of nitric acidis used to insure the dissolving ofl substantially all substancesYpresent.

To prevent precipitation of uranium and other Vforeign products duringthe phosphate precipitation phase, it. is desirable, first, that theacidity of`the solution be adjusted to between approximately 0.50 normaland 2.0 normal, and, secondly, that sulphate ions or other complexingions be present in the solution.V With respect to the acidity of thesolution, it has been found that an approximately onenormal acidsolution is 'most suitable for precipitation of the plutonium andbismuth as phosphates while leaving the uranium 'and other foreignproducts in solution. Belowv approximately 0.50 normal acidconcentration, uranyl phosphate tends to precipitate and befthrwn downwith the plutonium and bismuth phos# nitrate hexahydrate) in order tokeep the loss of element- Precipitation of uranium and other foreignproducts as phosphates is further prevented by the presence ofcomplexing ions such assulphate ions i'n the solution, which may beintroduced by the sulphuric acid used to acidify the solution. Usually,however, the solution willV bef adjusted tothe proper acid contentwithout sufficient sulphate ions having been introduced to preventprecipitation ot` the uranium phosphate. In such instances, sources ofsulphate ions other than sulphuric acid, such as NazSOr, NaHSOl, orother sulphate salts may be used for this purpose. It is believed thatthe sulphate ions form a complex with the uranyl ion which substantiallyprevents the latter from precipitating as an insoluble phosphate. Othercomplexing ions that may be used for this purpose are phosphate ions,P0473, in' high acid concentrations, carbonate ions, C03-2, in low acidconcentrations, or other elements or radicals that will complex theuranyll ion, UO2+2, and thereby substantially preventV the formation ofinsoluble uranyl phosphate.

As plutonium is generally present in relatively loul concentrations, acarrier such as bismuth phosphate is used to carry the plutoniumphosphate out of solution. Bit?v ions may be introduced into thesolution by the addition of bismuth nitrate orother bismuth compoundpreferably in acid solution. The concentration of Bi+3 ions may be'varied quite widely, depending upon the particular concentration ofplutonium in the solution. Usually, 25 mg. of Bi+3 per l0 cc. ofsolution is sufcient. When plutonium is present in tracer amounts it hasbeen found that a concentration of mg. of Bi+3 per l0 cc. of solutionwill precipitate with phosphate ions and carry substantially all of theplutonium out of solution.

An excess of phosphoric acid is added to the solution topr'eci'pitatethe bismuth, plutonium, and minor amounts of fission products asinsoluble phosphates. While the concentration of the phosphate ion maybe varied greatly, it has been found that generally the higher theacidity of the solution, the greater should be the concentration of thephosphate ion. A phosphoric acid concentration `in the solution ofapproximately 0.4M to 0.8M is usually suitable. However, Where phosphateions are also used to complex the UO2++ ions, higher concentrations ofphosphoric acid in the solution Such as concentrations of between 1M to1.5M are necessary to precipitate the bismuth and plutonium. It isnecessary that the solution be of the proper acidity and/ or that ithave a suitable amount of sulphate ions or other complexing ions presentwhen the phosphoric acid is added, otherwise some of -the uranium willbe precipitated as an insoluble uranyl 6 tion may be heatedduringor'a'fter the addition of the phosphate and bismuth ions.Preferably,rthe solution is heated for approximatelyan hour atl about 75C., whereupon a substantially complete precipitation of the bisinuthphosphate takes place, carrying with it substantially all of theplutonium phosphate as well as minor amounts of fissi-on products.

There is a tendency for the nitric acid used for dissolving the neutronirradiated uranium to oxidize a small portion of the plutonium to thephosphate-soluble state. To insure the presence of plutonium in theresulting solution in a reduced state, it may be desirable to add smallamounts of a reducing agent to said solution thereby making certain thatsubstantially a-ll of the Pu present will be in the phosphate insolublestate. Reducing agents such as salts of ferrous iron, hydrogen peroxide,hydrazine, or the like, may be used for this purpose. Although wegenerally prefer to have some reducing agent present at this stage as aprecautionary measure, for the purpose indicated, this preliminary stepfor the separation of uranium is not to be confused with the subsequentprocedure for the separation and concentration of plutonium, which wecall the oxidation-reduction separation.

The fission products that are carried out of solution with the plutoniumand bismuth phosphates 'are' principally radioactive zirconium andcolumbium phosphates. Minor amounts of other fission products, such ascerium and lanthanum, which form phosphates soluble in `acid solution,may be carried down to some degree with the bismuth phosphate. As it isdesirable to lower the amount of radioactive fission products (which areemitters of beta and/ or gamma radiation) and particularly thephosphate-soluble radioactive fission products, present in or carried bythe insoluble phosphates, the original solution may lhave added to itbefore the introduction of the phos# phate ions, i. e., before thebismuth phosphate precipitation step, small amounts of holdbackcarriers, which are preferably non-radioactive isotopes of the fissionproducts and particularly non-radioactive cerium and lanthanum. Theholdback carriers dilute the phosphate soluble, radioactive fissionproducts with the inctive form of such products so that the phosphatesoluble fission products which are carried by the bismuth phosphateprecipitate will include both the radioactive and inactive forms of thefission products, thereby to that extent decreasing the amount ofphosphate soluble radioactive' fission products carried by the bismuthphosphate.

The following tables set forth the percent of 94 carried by the bismuthphosphate precipitate, when different variables such as acidconcentration, sulphate ion concentration, temperature, time of heating,etc., are varied one at a time, in order to determine the conditionsthat will be most favorable for the separation of 94. The tables aregiven by way of illustration and not to limit the invention. Unlessotherwise specified the conditions of the experiments below were asfollows: 20%

UO2(NO3) 2.6H2O

1 N HNo or nzsor, @36M HgPor, 25 mg. Bi+3 per 1o cc., temperature ofprecipitation 95 C., .time of heating during precipitation l hour, andtracer amounts of 94.

Table II EFFECT OF VARIATION IN HaPOt CONCENTRATION HzP 04 concentration(Mol ar .54 .36 .18 .12 .09 .06 .03 1 N HNOS-Percent 94 1n 7 Table 1nTIME F HEATING AT 95 C.

Timeat95o 5 1o 15 2o so so 2' 5 s min. min. mln. min. mln. mln. hrs.hrs. hrs.

1 N HNO-Percent 94 in BiPO4 97 99 97 99 100 1 N H -Percent 94 in B :S04iPOr 98 99 99 99 99 Table IV VARIATION IN ACID CONCENTRATION Acidnormality (HQSO orHNOg 1.00 1.25 1.50 HNOS-Percent 94 in Table V EFFECTOF VARIOUS PRECIPITATION TEMPERATURE FOR ONE HOUR Temperature 25 O. 40C. 60 C. 80 C. 95 C.

1 N HNOa-Percent 94 in BiPO slight pe- 53 97 100 99 1 N HaSOi-Pereent 94in BiPO( 55 85 98 99 99 Table Vl VARIATION IN AMOUNT OF BISMUTHCARRIER-IN l N HiSO4 ONLY MgBw/iocc 25 2n 15 10 Percent 94mB1Po4 99 9s9s 9G Table VII VARIATION IN UO2(NO3)2.6H2O CONCENTRATION IN 1 N HzSO4SOLUTION ONLY Percent U02(NO5)2.6H2O 0. 0 10. 0 12 17. 6 20. 0 22. 4Percent 94 in BiPO4 99 100 100 99 98 95 Table VIII COBINATIONS 0F HNO,H2504, AND NmSOi Hi S 0 4 normality HNO normality NazS04 normalityPercent 94 in BiPOi The following example is given to aid in theexplanation of the invention and is not intended to limit the inventionto the details described therein.

EXAMPLE Il 9.4 ce. of solution is prepared containing 2.5 gm. ofUO2(NO3)2.6H2O containing 94, l cc. of l0 N HzSO4 and 25 mg. of Bi+3 (58mg. Bi(NO3)a.5H2O). 0.6 cc. of 6M HaPOi is then added. The solution isheated to 95 C. and maintained at this temperature for 1 hour. The BiPO4carrying the 94 is separated from the solution by filtration orcentrifugation, and, for purposes of analysis, dissolved in 'about l cc.concentrated HC1. The HCl solution is diluted to about 10 cc. and the 94precipitated using LaFa as carrier, in the manner described above. By Ythis method, consistent recovery of 94 to the extent of about 98% isreadily obtained.

Generally speaking, the BiPOi precipitate will also remove about 17% ofthe gamma activity due to fission products( originally present in A theneutron irradiated uranium mass. This activity corresponds to aboutnone- CFI y 90C. for Ifrom '-ve minutes to 'two'hours` or longer.

third of the total Zr, Cb fission activity present in the originalmaterial. The final LaFa precipitate contains about 5% of the gamma andabout 8% of the betafission activity present in this same material.

The bismuth phosphate precipitate obtained from the phosphateprecipitation phase includes plutonium and minor amounts of fissionproducts as phosphates. Substantially all of the uranium and majoramounts of fission products have been removed and the plutonium is readyfor further purification by the oxidation-reduction phase to bedescribed.

The oxidation-reduction separation alone may be used to separateplutonium from substances of the kind pres'- ent in neutron irradiateduranium, including uranium. Where the plutonium is present with suchsubstances, as where neutron irradiated uranium is treated directly, theoxidation-reduction separation is somewhat less eicient than where theplutonium has been subjected to a preliminary separation treatment whichremoves most of the uranium and fission products, in that the uraniuminterferes with the reactions by which the plutonium is separated. Theuse of a preliminary phosphate precipitation separation in conjunctionwith the oxidationreduction separation provides a complete process whichis particularly effective in obtaining plutonium in a pure state,especially where the original source of the plutonium is neutronirradiated uranium.

In carrying out the oxidation-reduction phase, the particular form inwhich the plutonium is to be treated is dependent somewhat upon theconcentrations of plutonium in the original solution. Where theplutonium is present in relatively high concentrations, that is, aboveapproximately 800 mg. per liter of solution, the plutonium will be inthe form of an'insoluble plutonium phosphate precipitate. Where theconcentrations of plutonium are low, `as for example, 10 mg. per literof solution, the preliminary phosphate precipitation phase will yieldthe plutonium as an insoluble phosphate carried by bismuth phosphate.

However, as the concentrations of plutonium in neutron irradiateduranium are normally so low as to require the use of a carrier such asbismuth phosphate, the` material to be treated by theoxidation-reduction phase is usually bismuth phosphate carryingplutonium phosphate and minor amounts of fission products in the form ofphosphates.

The bismuth phosphate precipitate is dissolved in an excess ofconcentrated acid, such as ten normal nitric acid. The solution isdiluted with water to a suitable -acidity for oxidation of theplutonium, such dilution being dependent somewhat upon Athe oxidizingagents used; for most oxidizing agents, the acidity should be betweenabout two normal and ten normal. If desired, the oxidizing agent oragents can be added before or after dilution.

The oxidizing agent or agents in amounts sufficient to oxidize theplutonium and convert it to its phosphate soluble state are added to thesolution. While any oxidizing agent having a potenti-al greater than 1.0volt will tend to oxidize the plutonium, oxidizing agents that have beenfound to be most suitable for this purpose are dichromates such asK2Cr207. Na2Cr2O7 or the like,

peroxydisulfate in the presence of silver nitrate.

Where dicromates, such as KgCrzO7 or NaCr2O7, are used, the yacidity ofthe solution should Ibe between 2N and 6N. The dichromate is added inthe amount of .OOlM to .1M and is permitted to react for one-half hourto four hours at between 50 C. to 95 C. Where the solution has anacidity of 2N and a kdichromate in the amount of .()GlM is used, thesolution is preferably heated for approximately one hour at 75 C. Wheresodium bismuthate, NaBiO3, in the amount of .005M is used, the solutionshould be between 3N and 7N acidity and the solution vmaintained at atemperature of from A50" C. to

nitric. acid therefore converts all these substances to solublenitrates. The hydroxide treatment may be used at'any point in either thephosphate precipitation phase or oxidation-reduction phase in which theplutonium is present with bismuth phosphate.

After the plutonium has been treated by the oxidationreduction phase inwhich bismuth phosphate is used as a carrier, the plutonium and bismuthare obtained as phosphates. The plutonium may be separated from thebismuth by any one of several methods, For example, the bismuthphosphate carrying plutonium phosphate may be dissolved in an acid suchas hydrochloric acid and a carrier, such as lanthanum fluoride or thelike, may be precipitated from the solution to carry down the plutoniumaway from the bismuth in the manner already described. The carriershould be of such a nature that it is either required in lesser amountsthan bismuth phosphate to 4carry a given amount of plutonium or suchthat it is more soluble in certain solvents than is bismuth phosphate sothat it may be dissolved in such a small amount of solvent that theplutonium will be present in relatively high concentrations andconsequently, may be precipitated therefrom as a substantially pureplutonium compound. Where bismuth phosphate is converted to bismuthhydroxide at one or more points throughout the process, the volume ofthe solution may be decreased to a point Where plutonium will be presentin suchlliigh concentrations that it may be separated from the bismuthby being precipitated as a plutonium compound without the use of anadditional carrier.

EXAMPLE III As a further specific embodiment of the invention, andreferring particularly to the drawing, the following is given by Way ofillustration. A bismuth phosphate precipitate 2, carrying plutonium andminor amounts of lission products, was obtained from a preliminaryphosphate precipitation phase as applied in the manner described aboveto a solution 1 comprising 1000 gms. of uranyl nitrate hexahydrate whichhad been subjected to an extended neutron bombardment. ln carrying outthe 'preliminary phosphate precipitation step l, l gms. of Bit3 wasprecipitated as bismuth phosphate from 4 liters of 20% U02 (NO3)2.6H2O1N H2804 in the presence of 0.36M phosphoric acid and the bismuthphosphate precipitate 2 was then washed with an acid mixture of 1N inHNOs and 0.4M in H3PO4. The waste filtrate 3 was discarded.

In the rst cycle Il of the oxidation-reduction phase, the bismuthphosphate precipitate 2 was dissolved in 10N HNOS, the acidity reducedto 6N HNOS by dilution, and the solution made 0.1M` in KaCrzO'z. Theplutonium was oxidized by heating this solution at 95 C. for 2.25 hours.The solution was then diluted to 1N acidity by adding distilled water`and sufficient H3PO4 was added to bring the concentration of HsPO4 to.05M. The solution was heated to approximately 90 C. to throw down thebismuth phosphate precipitate 4 which was then removed by filtration anddiscarded. The plutonium in the filtrate 5 was reduced by passing in arapid stream of SO2 gas for 5 minutes and allowing the solution to standfor approximately 1 hour. Bit'` was then added as a N HNOs solution tomake the Bit3 concentration 0.8 mg. per cc. (This is one-thirdV theamount of Bi+3 used in the phosphate precipitation phase and serves toreduce the volume in the subsequent cycle by a factor of 3.) It wasfound that the Bi+3 did not precipitate completely as BiPOfi, probablybecause of complex formation between the Cr+3 and HaPO4. Therefore, theHaPOi concentration was adjusted to 70.10M and the solution heated to 90C. The bismuth phosphate carrying plutonium precipitated substantiallycompletely was washed and the precipitate V6 was separated byfiltration.

The waste filtrate 7 was discarded.

substances present in the original uranyl nitrate hexah'y drate solutionshowed approximately 95% recovery of the plutonium and a re'ductionfinfission product activity associated with"the plutonium Vby a factor of`103. As this amount of fission product activity was still in excess ofthat desired, the above described oxidation-reduction phase was carriedthrough a second and third cycles to obtain a reduction in the amount offission product activity associated with the plutonium by a factor ofl0".

In the second cycle III, the bismuth phosphate precipitate 6 wasdissolved in nitric acid and treated in the same manner as in the firstcycle, except for a reduction by a factor of 3 in solution volumes andamounts of reagents used. Thus, the bismuth phosphate precipitate 6 wasdissolved in nitric acid, the plutonium oxidized to its phosphatesoluble oxidation state, and a precipitate obtained by the addition ofphosphoric acid. The waste precipitate 8, containing bismuth phosphate,was separated from the solution by filtration and discarded. Thefiltrate 9, Icontaining the plutonium, was treated with a sulphurdioxide to reduce the plutonium. The plutonium was then precipitated,the precipitate 1l) being removed from thefsolution by ltration, Thewaste filtrate 11V was discarded. This completed the second cycle. Y

The third cycle IV was carried out in the same manner as the secondcycle except for a further reduction by a factor of 3 in the volumes ofsolutions and amountsof reagentsV used. The precipitate 10, containingthe plutoniurn, was dissolved and the plutonium oxidized to itsphosphate soluble oxidation state. The solution was diluted toobtain'the Waste bismuth phosphate precipitate 12`which Wasremoved fromthe solution by filtration and discarded. The filtrate 13 was treatedwith sulphur dioxide to reduce the plutonium. Upon precipitation, the

- plutonium was c ontained in the bismuth phosphate precipitate t4 as arfinal purified product, the plutonium being precipitatedwith llO mg. ofBit3 as bismuth phosphate, which was the smallest amount of Bit3 thatcould be used to separate the plutonium from this solution,

Vwhich was 0.36M in H3PO4. The waste filtrate 15V was discarded. Y Y

An analysis of the fractions containing plutonium in the phosphateprecipitation phase and in the three oxidation-reduction cycles is givenin the followingtable:

l Total absorber thickness including sample and counter window, etc.were Vgenerally maintained at w10 ing/cm2 This thickness appliesparticularly to precipitate fractions containing the plutonium.However-,such sample 'thicknesses could not be maintained with many ofthe tlltrates.

2 Counted through 2.75 grams ci lead per square centimeter.

Counted through lOgrams of lead per square centimeter.

Table X is a material balance chart showing plutonium recovery andfission product activities in various fractions as Shown in the drawingfor a phosphate precipitation phase and two oxidation-reduction cycles.

In this table, the columns denoted as A refer to the fractions in termsof counts per minute `per gram of UO2(NO3)2.6H2O. The columns denoted asB refer to the radioactivity in terms o f percent of the total fraction.The totalrecovery of plutonium is indicated in the last line ofthe tabledesignated'as P.

(le+4 may be used in an amount of .02M with a solution having an acidityof from 2N to 7N and maintained at 75 C. for from one to two hours.Where Ce+4 is lused with dichromate such as K2Cr20 the amounts used arepreferably .002M to .02M of Ce+4 with .01M to .1Mjof K2Cr207. Thesolution is adjusted further to 2N to 10N acidity and maintained forfifteen minutes to two hours at 50 C. to 90 C. The foregoing rangesgiven as to amounts, acidity, time, and temperature are the preferredVlimits where the particular oxidizing agents are used.

The plutonium is thus oxidized to a higher oxidation state in which itremains soluble in the presence of phosphate ions. Also present in thesolution, which is between 2N and 10N acidity, are dissolved bismuthphosphate and fission products. The solution is diluted or neutralizedto approximately one normal acidity to precipitate bismuth phosphate. Ifdesired, phosphoric acid in suicient amount to bring the solution tobetween 0.1M to 0.8M in Ha-PO, may be added to aid in precipitating thebismuth phosphate. The bismuth phosphate will be precipitated and willcarry out of solution certain radioactive fission products such aszirconium, columbium, and, possibly, cerium and some others. Theplutonium, of course, remains in solution as it is in a phosphatesoluble state.

The bismuth phosphate precipitate containing the phosphate insolublefission products is removed by convenient means, such as filtration orcentrifugation, and discarded. The solution containing the plutoniumalso contains certain iission products which are soluble in the presenceof phosphate ions. To separate the plutonium from such iission products,the plutonium is converted to its phosphate insoluble form, that is, itslower oxidation state, by the use of reducing agents. Any strongreducing agent may be used, although reducing agents which have beenfound particularly suitable for this purpose are H2O2, ferrous nitrateor ferrous lammonium sulfate, hydrazine, SO2, or H2O2 and ferrousnitrate. The reducing agents are permitted to react for approximatelyone hour at a temperature of from 50 C. to 75 C. While an excess of thereducing agent may be used, such excess should not be too greatv becausein certain instances an excess of the reducing agent may decrease thecarrying power of bismuth phosphate with respect to plutonium.

The solution now contains plutonium in its reduced state, that is, onein which it is insoluble in the presence of phosphate ions. Additionalphosphoric acid may be added to make the phosphoric acid concentrationbetween 0.1M to 0.8M. Bit3 in the form of bismuth nitrate in acidsolution is introduced in several portions to form a bismuth phosphateprecipitate which carries with it the insoluble plutonium phosphate. Tofacilitate the precipitation of the bismuth phosphate the solution maybe heated, as for one hour at approximately 75 C. The

.plutonium carried by the bismuth phosphate precipitate is thussubstantially completely separated from fission products and otherforeign products.

Where the oxidation-reduction separation as -above described is used incombination with the preliminary phosphate precipitation step, at thecompletion of one oxidation-reduction cycle there is a ydecrease in theradioactive fission products associ-ated with the plutonium by a factorof from 102 to 104. To obtain a reduction in the radioactive ssionproducts by a factor of more than 107, the oxidation-reduction phase maybe repeated' from two to four times.

Where the plutonium is initially present in such high concentrationsthat no carrier need be used in the phosphate precipitation phase, thematerial to be treated by the oxidation-reduction phase is an insolubleplutonium phos cept that a carrier, such as bismuth phosphate,.neednot'` be used, at lleast in the first oxidation-reduction separation: A

In the preliminary phosphate precipitation phase, particularly wherebismuth phosphate is used as a carrier, certain foreign products such aszirconium and columbium, and possibly barium or other substances, arecarried down with fthe plutonium phosphate.

To decrease the amountV of fission Vproducts carried down with theplutonium phosphate precipitate, an oxidizing agent such as Cr207= maybe added to the uranyl nitrate hexahydrate solution to oxidize theplutonium and convert it to its phosphate soluble state. After theplutonium has been oxidized, the phosphate precipitation separation iscarried out as described above, that is, the solution is acidied toapproximately one normal acid content, the proper amount of sulphateions is added, and bismuth precipitated with phosphoric acid. As theinsoluble bismuth phosphate is precipitated, it carries with it certainforeign products, particularly zirconium and columbium, and possiblybarium sulphate. The plutonium is not precipitated as it has beenoxidized to its phosphate soluble state. The bismuth phosphateprecipitate containing a portion of fission products carried down withit is removed and discarded. The plutonium in the filtrate (whichcontains substantially all the uranyl nitrate originally present) isreduced by a suitable reducing agent, such as Fe+2 ions, to convert theplutonium to its phosphate insoluble form. The solution containing thedissolved plutonium may then be treated again in accordance with thedescribed phosphate precipitation phase which comprises introducingbismuth ions to yield a bismuth phosphate precipitate which will carrythe insoluble plutonium phosphate out of solution, and thus separate theplutonium from uranium and the major portion of the tission productswhich form soluble phosphates. Such a'direct oxidation-reduction phaseremoves a relatively large portion of the phosphate-insoluble foreignproducts which would otherwisebe associated with the plutonium after thephosphate precipitation phase.

It is desirable when carrying out the oxidation-reduction separation inthe complete process in which the preliminary bismuth phosphateprecipitation separation is used in conjunction with theoxidation-reduction separationto maintain the volume of solution treatedas small as possible. This may be particularly desirable where theprocess is carried out on a large scale. The principall reason for thelarge volume of solution ordinarily required after the plutonium isassociated with bismuth phosphate is the insolubility of bismuthphosphate which makes necessary the use of either largeamounts or dilutenitric acid or of small amounts of concentrated nitric acid to dissolvethe bismuth phosphate and plutonium phosphate. Where strong nitric acidis used, the solution must be subsequently diluted to a relatively largevolume to obtain the proper acidity for treatment.

To decrease the volume of solution to be treated, the bismuth phosphateprecipitate (carrying plutonium phosphate and minor amounts of foreignproducts which are obtained from the phosphate precipitation phase orfrom the oxidationreduction phase) may be converted to a form which ismore soluble in nitric acid and thus may be dissolved by a relativelysmall volume of solvent. This may be done by digesting the bismuthphosphate precipi tate with a caustic alkali solution, such as potassiumhydroxide or sodium hydroxide, thereby forming insoluble bismuthhydroxide, Bi(OH)a. The bismuth hydroxideis Washed with Water to removepotassium phosphate and is then dissolved in nitric acid. As bismuthhydroxide is much more soluble in nitric acid than is bismuth phosphate,the volume of the bismuth nitrate solution to be treated will beconsiderably decreased by this step. The plutonium and foreign productsassociated with the bismuth phosphate are acted upon by the causticalkali solution in the same manner as the bismuth phosphate, and thesubsequent treatment of the bismuth hydroxide with l Counted through2.75 grams of load per square centimeter. 2 Counted through 10 grams oflead per square centimeter'.

Where the oxidation-reduction separation is carried out Without thepreliminary phosphate precipitation separation, the presence of uraniumwill retard the react1ons to a certain extent. The plutonium,nevertheless, can .be separated from substantially all of the uraniumand major amounts of fission products by the oxidation-reductionseparation alone, if desired. Where uranyl nitrate hexahydrate istreated directly in accordance with the oxidation-reduction separationdescribed above, it is desirable to repeat such separation procedure oneor more times to still further decrease the amounts of radioactivefission products associated with the plutonium. In such case, theprocess may be considered as comprising an oxidation-reduction cycle inthe presence of uranium and one or more further oxidation-reductioncycles in the absence of uranium.

While no particular details have heretofore been given as to thepreparation of the neutron irradiated mass which serves as the source of94 in the separation process of the present invention, among the methodswhich are suitable for preparing such la mass may be mentioned, by wayof illustration, the bombardment of metallic uranium for about 40 dayswith neutrons produced by the action of 12 m. e. v. deuterons onberyllium, followed by aging for a period of about 60 days to permitsubstantially complete conversion of 93 to 94.

The above detailed description is given for purposes of illustration andthe invention is to be limited only by the scope ofthe appended claims.

What is claimed is:

l. A method of separating plutonium from phosphate soluble foreignproducts yand phosphate insoluble foreign products which -comprisesadding bismuth ions to a solution containing plutonium in a valent statenot above 4, phosphate insoluble foreign products, and phosphate solubleforeign products, thereby precipitating as phosphates said bismuth, saidplutonium, and at least a portion of said phosphate insoluble foreignproducts, removing said precipitate from said solution, dissolving saidprecipitate in an aqueous acid medium, oxidizing said plutonium to anoxidation state greater than 4 and thereafter contacting the resultingsolution with phosphate ions.

2. A method of separating plutonium from substances present in neutronirradiated uranium including uranium and tission products whichcomprises forming a solution of neutron irradiated uranium, addingbismuth ions to said solution, maintaining said plutonium in anoxidation state not greater than 4, contacting said solution withphosphate ions thereby precipitating as phosphates said bismuth, saidplutonium, and Ia portion of said fission 14 products from saidsolution, dissolving said precipitate in an aqueous acid medium,oxidizing said plutonium to an oxidation state greater than 4 andprecipitating said fission products as insoluble phosphates from saidplutonium by contacting the resulting solution with phosphate ions.

3. A method of separating plutonium from substances present in neutronirradiated uranium including uranium and fission products whichcomprises dissolving neutron irradiated uranium, in yan acid medium toproduce a solution of between 0.50 N and 2 N acid content, addingbismuth ions and phosphate ions to said solution which contains areducing agent to maintain plutonium in its reduced state, precipitatingsaid bismuth, said plutonium, `and minor amounts of said fissionproducts, as phosphates by contacting the solution with phosphate ions,removing said precipitated substances from said solution, dissolvings-aid precipitated substances in an aqueous acid medium whose acidity isgreater than that of the solution from which said substances wereprecipitated, oxidizing said plutonium to its phosphate soluble state,diluting said solution to approximately one normal acidity,precipitating said bismuth and some of said fission products asphosphates by contacting the resulting solution with phosphate ions, andremoving said precipitate from said phosphate soluble plutonium.

4. A method of separating plutonium from substances present in neutronirradiated uranium including uranium 'and fission products whichcomprises dissolving neutron irradiated uranium in nitric acid to form auranyl nitrate hexahydrate solution, adjusting the acidity of saidsolution to between 0.50 N and 2 N acid content, adding bismuth ions andphosphate ions to said solution which contains a reducing agent tomaintain plutonium in its reduced state, precipitating said bismuth,said plutonium, and minor amounts of said fission products as phosphatesby contacting said solution with phosphate ions, removing saidprecipitated substances from said solution, dissolving said precipitatedsubstances in an acid solution, oxidizing said plutonium to itsphosphate soluble state, diluting said solution to approximately onenormal acidity, precipitating said bismuth and some of said ssionproducts as phosphates, by contacting said solution with phosphate ions,removing said precipitate from said phosphat soluble plutonium, reducingsaid plutonium to its phosphate insoluble state, 'adding bismuth ions tosaid solution, and precipitating said bismuth and-said plutonium asphosphates from said ssion products which are in solution.

5. In a process for the separation of plutonium from a solutioncontaining that element together with phosphate insoluble foreignproducts, the s-tep which comprises contacting 'a solution containingplutonium in a valent state greater than 4 and said foreign productswith phosphate ions in the presence of bismuth phosphate.

References Cited in the le of this patent UNITED STATES PATENTS Fermi etal. July 2, 1940 OTHER REFERENCES

5. IN A PROCESS FOR THE SEPARATION OF PLUTONIUM FROM A SOLUTIONCONTAINING THAT ELEMENT TOGETHER WITH PHOSPHATE INSOLUBLE FOREIGNPRODUCTS, THE STEP WHICH COMPRISES CONTACTING A SOLUTION CONTAININGPLUTONIUM IN A VALENT STATE GREATER THAN 4 AND SAID FOREIGH PRODUCTSWITH PHOSPHATE IONS IN THE PRESENCE OF BISMUTH PHOSPHATE.